Continuous monitoring of the mass concentration of particulate matter from source emissions is becoming a growing requirement within the framework of clean air regulations in both the US and abroad. Of great interest is the desire to measure a particulate mass concentration in a form that simulates the source emission after it has equilibrated to ambient air conditions. This form of particulate matter is known as Total Primary Particulate (TPP) and is comprised of particulate matter that can be removed directly from a source through the use of filterable extraction plus particulate that can be created by condensation at a reference temperature from the remaining fluid stream.
The extraction, transport, conditioning and measurement of a sample from a source are critical processes necessary to provide consistent results and low downtime for service. Each of these processes gives rise to specific challenges, and with proper design, can be overcome and reduce instrumentation service intervals.
Through the past few decades the Unites States Environmental Protection Agency (EPA) has promulgated at least 15 reference methods for measuring particulate matter from source emissions. The root cause of this plethora of methodology stems from the varying conditions in which the EPA justified modifying the originating method (e.g., Method 5). As a result of these varied methods, instrumentation manufacturers have taken advantage of their ability to install and calibrate (correlate) a surrogate particulate matter measurement (e.g., light scattering, opacity, probe electrification) to one of the multitude of reference methods, which can vary in accuracy by an order of magnitude.
The concept of measuring TPP is not new to the art. A synonym for this methodology is known as dilution tunnel sampling whereby a sample is extracted from a source and diluted with filtered ambient air so that conventional ambient air samplers or analyzers can be used to measure the diluted concentration. By simultaneous measurement of the magnitude of dilution (dilution ratio), the measured concentration would be multiplied by the dilution ratio to calculate the actual concentration within the source. In the last 20 years prior art has documented this approach:                i) SAMPLING, ANALYSIS, AND PROPERTIES OF PRIMARY PM-2.5: APPLICATION TO COAL-FIRED UTILITY BOILERS. DOE AWARD #: DE-FG2699-FT40583, FEBRUARY 2003; DILUTION TEST METHOD FOR DETERMINING PM2.5 AND PM10 MASS IN STACK GASES. ASTM WK8124˜2008;        ii) SOURCE CONTRIBUTIONS TO ATMOSPHERIC CARBON PARTICLE CONCENTRATIONS. G. R. CASS, CALIFORNIA INSTITUTE OF TECHNOLOGY, 1992; and        iii) CONDITIONAL TEST METHOD 039, MEASUREMENT OF PM2.5 AND PM10 EMISSIONS BY DILUTION SAMPLING, USEPA, JULY 2004.        
Particulate monitoring can be a complex process. Although reference methods are capable of operating within complex source emission environments, the operating period can be somewhat short-lived. Frequent cleaning of equipment and recovery of sample may be necessary.
These types of steps for a continuous monitor are unacceptable and therefore innovative approaches are required to extract, handle and condition a sample for the purpose of measurement. For example, the USEPA often requires the use of in-stack cyclones, which will separate, through inertia, the particles of interest for measurement and collect and remove larger particles that are not of interest. Such cyclones can only collect and remove unwanted particulate for a finite period of time. However, use of this approach is unacceptable from a service perspective for a continuous monitoring system whether it is for an in-stack cyclone or a post-diluted sample cyclone.
Another complexity for of particulate matter monitoring systems is that of the source environment. Although highly corrosive environments are expected, the larger challenge is to design a system such that it may be utilized in a water-saturated environment as well as very hot, dry environments. In a saturated environment, both particulate matter and water droplets less than approximately 40 micrometers need to be collected based on the theory that a droplet will have a 4:1 reduction in size and, when dried, will become a 10 micrometer particle—which may be of interest.